About the Birth & Evolution of a Star
Everyone who looks at the night sky is awed by the stars. Since pre-history, people have studied and contemplated the stars. It wasn't until this century and the use of advanced science that cosmologists and astrophysicists were able to understand the nature of stars--what they are made of and how they form.
Function
The physics behind stars are simple. Stars are born, live and die based on gravity, pressure and heat. All stars begin as molecular clouds--giant gas clouds of mostly hydrogen. When the cloud reaches are certain size, gravity will cause it to start collapsing. This causes the particles in the cloud to increase in speed and temperature. Eventually, the molecular cloud collapses into a very hot state called plasma that gives off light and heat. The plasma cloud is called a protostar. The protostar will continue to collapse. If it is massive enough, the heat and pressure will eventually cause the nuclei in the center of the protostar to ignite. This nuclear fusion signals the birth of the star and the main-sequence stage will begin. The star will continue burning through its fuel. When it has consumed much of its hydrogen, the star will begin to fuse helium. If the star is average size like our sun, it will begin its red giant phase, where it will increase in size and decrease in temperature. Stars die in a variety of ways depending on their mass. Average size stars either end when their fuel runs out as a stable white dwarf or die out violenty in a nova explosion.
Time Frame
The following timeframe assumes an average star of about 1 solar mass (size of the sun):
Molecular cloud phase: 1 billion years
Protostar phase: 1 billion years
Main-Sequence phase: 10 billion years.
Red Giant phase: 1 billion years
Collapse: 1/2 billion years
Features
Stars vary in color and size. Non-cosmologists often judge a star by its brightness in the night sky. This can be misleading, as stars' distance from the viewer can vary dramatically. The brightest objects in the night sky are actually planets, which are much smaller than the dim stars around them but make up for that by their proximity to Earth. The sun is an average star. There are stars in our galaxy many orders of magnitude larger than the sun. See the image below for comparison.
Significance
Without stars, the universe would be a very cold, dark place. There would be no elements other than hydrogen, as all other elements are created by fusion in the core of stars. Over the billions of years of the universe's existence, many stars have ejected these elements in explosions called supernovas. These heavier elements have formed planets such as the Earth. All the elements that make up carbon-based life were also formed in the core of the stars.
Identification
Stars are identified by their places in the night sky. Using the naked eye, people have catalogued the stars for centuries. These patterns are called constellations. Examples of well-known constellations visible in the Northern Hemisphere are Ursa Major (the Big Dipper) and Orion. Though most people now longer believe in the myths told about the star constellations, astronomers still refer to them as markers in the night sky.
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